The present invention relates to a beam generating apparatus for use in an apparatus for fabricating semiconductor devices for surface-treating substrates in vacuum chambers and forming layers on the substrates, and to a method for fabricating semiconductor devices using the apparatus for fabricating semiconductor devices.
In the beam generating apparatus, gas is introduced into the vacuum chamber, and the gas is heated thereby to be cracked to emit molecular beams, or the gas is plasmatized thereby to be activated to emit activated beams.
In semiconductor fabricating techniques, activated beams from the beam generating apparatuses are used to clean or etch the surfaces of semiconductor substrates, or form semiconductor layers and dielectric layers on semiconductor substrates of high quality. Accordingly in an apparatus for fabricating semiconductor devices the beam generating apparatus does a most important role which influences properties of fabricated semiconductor devices.
The beam generating apparatus is provided in the high-vacuum chamber. Accordingly vacuum sealing of the beam generating apparatus is one of the most important problems.
Generally it is difficult to vacuum seal two members which are exposed to high temperatures above 1000.degree. C. This is because limited materials are heat-resistant and come out little gas. For example, insulators, such as quartz, alumina, PBN (pyrolitic boron nitride), sapphire, etc., metals of high melting points, such as tantalum (Ta), molybdenum (Mo), tungsten (W), etc., and semiconductors, such as carbon (C), silicon carbide (SIC), etc., have such properties.
On the other hand, a stainless steel tube, as of SUS316L or others, is used for the introduction of gas into the vacuum chamber. Most of the above-mentioned materials cannot be welded with stainless steel. Even if they can be welded to stainless steel, members integrated by welding make the maintenance of the apparatus very difficult.
Means of providing seal rings between the two materials for the vacuum sealing is not suitable to seal the materials which are exposed to high temperatures because no sealing ring materials have heat resistance to about 1000.degree. C.
For the above-described reason, the conventional beam generating apparatus uses the vacuum sealing method of FIG. 1. This conventional beam generating apparatus is used as a radical/atomic beam source (a beam source in which gas is cracked by heating or activated by plasmatizing to generate activated beams), or a gas cell (gas source using MBE (molecular beam epitaxy) apparatus).
The conventional beam generating apparatus is mounted on a wall 48 of a vacuum chamber. On the wall 48 there is mounted a base flange 47 which is a base of the beam generating apparatus. A gas feed pipe 41 is attached to the base flange 47 for introducing gas. The gas feed pipe 41, and a beam generating cell 43 (which functions as a discharge tube in a beam generating apparatus for generating radical/atomic beams, and functions as a cracker to be heated by a heater in a beam generating apparatus for generating molecular beams) are interconnected by a heat resistance pipe 42 of quartz, alumina, PBN, Ta or others. An aperture plate 44 is provided in the beam generating cell 43. The aperture plate 44 has an aperture 49 for passing the activated beams. The aperture plate 44 is provided in the beam generating cell 43 by an aperture plate aligner 46 mounted on a support base 45.
In the conventional beam generating apparatus, as shown in FIG. 1, the gas feed pipe 41 and the heat resistant pipe 42 are vacuum-sealed by inserting one in the other (matching on their tapered surfaces) with their matching machining precision, and the heat resistant pipe 42 and the beam generating cell 43 are vacuum sealed by a pressure resulting from their matching machining precision. The beam generating cell 43 and the aperture plate 44 are vacuum sealed by pressing each other by the aperture plate aligner 46 screwed on the support base 45.
Another problem with the beam generating apparatus is a method of heating the beam generating cell. In the conventional beam generating apparatus, to heat a gas cell used in a gas source MBE, a Ta heater (not shown) is provided around the outer circumference of the cell so as to heat the gas cell by radiation of the Ta heater.
But the above-described vacuum seals have the following problems.
Firstly, the seals by the above-described insertion and pressure have poor air-tightness. In generating either of atomic beams and molecular beams, higher gas pressure is preferred to enhance the decomposition efficiency. But at high gas pressures, fed-in gas leaks at the vacuum seals, with a result that ratios of atoms or radicals which arrive at semiconductor substrates located in the vacuum chamber lowers, and reaction efficiency lowers.
Secondly, when the beam generating cell has high temperatures, thermal distortion of the constituent members further lowers the air tightness, and sometimes the constituent members are damaged.
The above-described heating method of the beam generating cell has the following problems.
Firstly, gas involved in the growth reacts with Ta of high temperature, with a result that the Ta heater tends to have wire breakages.
Secondly a heat capacity including that of the beam generating cell is generally so large that it is difficult to quickly response to raised and lowered temperatures.
Thirdly, since members near the heater have high temperatures, gasses come out from these members cause contamination.
Fourthly, generally the heating by radiation of a heater requires a heater support, a radiation shield, water-cooling pipes, etc., which make the structure complicated.
On the other hand, it is very important to semiconductor device fabricating techniques to form oxide layers of high quality on semiconductor surfaces.
Semiconductor oxide layers, e.g., silicon (Si) oxide layers, are used as gate oxide layers, inter-layer insulating layers, inter-device separation insulating layers of MOSs (metal-oxide-semiconductors) field-effect transistors.
The gate oxide layers are required being as thin as some nanometers to tens nanometers as MOS field-effect transistors are down-scaled. As the device is down-scaled, to suppress diffusion of undesired impurities, lower-temperature processes are important.
Under these circumstances, it is expected that a technique which can form at low temperatures semiconductor oxide layers having few interface traps, high dielectric strength, and high quality.
Conventionally the gate oxide layer of a MOS field-effect transistor is formed usually by thermal oxidation. The oxidation method includes dry oxidation containing no water in ambient atmosphere, and wet oxidation containing water in ambient atmosphere.
In these thermal oxidation methods, silicon substrates are positioned in a furnace core pipe of quartz, and the substrates are heated up to about 1000.degree. C. in acid ambient atmosphere of oxygen, water vapor, etc. to form oxide layers on the surfaces of the substrates. It is known that acid ambient atmosphere containing water vapor enhances oxidation speeds.
As semiconductor devices are down-scaled, for the prevention of changes of formed impurities distributions and crystallinities of semiconductors, the low-temperature fabrication of semiconductor devices are required.
The temperature reduction has been tried on the thermal oxidation. Even in high-pressure oxidation, unless silicon substrate are heated up to above 700.degree. C., oxide layers of required characteristics cannot be formed, and the oxide layers cannot be dense.
This will be because, although oxidizing materials diffuse in the oxide layers and associate with silicon atoms in the interfaces of the oxide layers, sufficient energy for the oxidation is not given.
For this reason, the following methods are necessary to form silicon oxide layers. A first method is for depositing a silicon oxide layer on a silicon substrate. A second method is for using oxidizing materials which sufficiently oxidize silicon even at low temperatures.
In the first method, the deposition of a silicon oxide layer can be conducted by CVD or sputtering but find it difficult to remove impurities in the interface of the oxide layer, and to form a dense oxide layers. Accordingly it is so far difficult to use this method for forming gate oxide layers of MOS transistors.
In the second method, in which an oxidizing material which sufficiently oxidizes silicon at even low temperatures is used, it can be proposed to add a catalyst for accelerating oxidation of the oxidizing material. But this method has a risk of adding undesired impurities. To overcome these problems, low-temperature oxidation using activation materials, such as oxygen plasma, anodic oxidation in plasma, oxygen radicals by UV radiation, etc., are being studied.
But high energy particles, such as ions and electrons generated in plasma, UV rays, soft X-rays, damage oxide layers, which results in deviations of a threshold voltage of MOS transistors, increases of leak currents, and decreases of switching speeds, etc. A method which uses oxygen radicals by UV radiation is prospective as a method which less damages oxide layers unless the UV radiation is directly applied to the substrates, but has a problem in terms of efficiency.
Thus methods for forming silicon oxide layers of high quality at low temperatures are still insufficient.